A laser is a device that produces an intense, narrow beam of light through stimulated emission, where excited atoms drop to lower energy levels and release identical photons. The result is coherent (in-phase) and monochromatic (single-wavelength) light, which is why lasers show up in AP interference problems.
A laser works because of stimulated emission. An atom sitting in an excited energy level can be 'tickled' by a passing photon into dropping to a lower level, and when it does, it releases a new photon that is an exact copy of the one that triggered it. Same energy, same wavelength, same phase, same direction. Chain that process through billions of atoms and you get a beam of light where every photon marches in step.
That gives laser light its two famous properties. It's monochromatic, meaning essentially one wavelength, because each photon's energy equals the exact energy difference between two atomic levels. And it's coherent, meaning the waves stay in phase with each other. Coherence is the property that makes stable interference patterns possible, which is exactly why every double-slit problem you'll see starts with 'a laser emits monochromatic light.' In Topic 7.7, that interference pattern gets a quantum twist. The bright and dark fringes describe where individual photons are more or less likely to land, so the pattern is really a probability distribution.
Lasers live in Unit 7 (Modern Physics) and connect directly to Topic 7.7, Wave Functions and Probability. The laser is the bridge between two big ideas in that unit. First, quantized energy levels explain where the light comes from, since stimulated emission only happens between specific atomic states. Second, the interference pattern a laser makes through two slits is the cleanest evidence that light behaves as a wave, and when you send photons through one at a time, the pattern builds up dot by dot according to probability. That's the wave function idea in action. On the exam, the laser is rarely the question itself; it's the tool that sets up interference, diffraction, and photon-probability questions.
Keep studying AP Physics 2 Unit 7
Stimulated Emission (Unit 7)
This is the mechanism inside the laser. A photon triggers an excited atom to emit an identical photon, so one photon becomes two, then four, then an avalanche. The 'se' in laser literally stands for stimulated emission.
Energy Levels (Unit 7)
Laser light is monochromatic because each photon carries exactly the energy gap between two atomic levels, E = hf. Quantized levels mean one specific transition, which means one specific wavelength.
Coherent Light (Unit 7)
Coherence means the light waves stay in phase with each other over time. Lasers are the textbook coherent source, and coherence is what lets a double-slit pattern hold still instead of washing out.
de Broglie Wavelength (Unit 7)
The double-slit setup that uses a laser for light can be rerun with electrons, which interfere according to their de Broglie wavelength. Same fringe math, different particle. That parallel is the heart of wave-particle duality in Topic 7.7.
The laser is almost always the setup, not the answer. The 2025 FRQ Q4 is the classic example: a laser emits monochromatic light toward two narrow slits a distance d apart, and you analyze the pattern of bright and dark fringes on a screen a distance L away. You need to apply the path-difference condition (d sin θ = mλ for bright fringes), predict how the pattern changes if d, L, or λ changes, and explain the pattern in terms of constructive and destructive interference. Topic 7.7 adds the quantum layer, so be ready to explain that when photons pass through one at a time, each photon lands at a single point but the bright fringes mark where photons are most probable to arrive. Multiple-choice questions may also ask why a laser is used at all, and the answer is coherence and a single wavelength, both consequences of stimulated emission between fixed energy levels.
A lightbulb emits photons by spontaneous emission, so the photons come out with many wavelengths, random phases, and random directions. A laser uses stimulated emission, so every photon is a clone: one wavelength, one phase, one direction. That's why a laser through two slits makes crisp, stable fringes while a flashlight makes a blur. If an exam question swaps a laser for white light, expect the pattern to smear into overlapping colored fringes.
A laser produces light through stimulated emission, where a photon triggers an excited atom to release an identical photon.
Laser light is monochromatic because each photon's energy matches one specific energy-level transition (E = hf).
Laser light is coherent, meaning the waves stay in phase, which is what makes stable double-slit interference patterns possible.
On the AP exam, the laser is the standard light source in interference problems, like the 2025 FRQ where laser light through two slits creates bright and dark fringes.
In Topic 7.7, the laser's interference pattern doubles as a probability map showing where individual photons are most likely to land.
A laser is a device that emits an intense, narrow beam of light through stimulated emission, where excited atoms drop between energy levels and release identical photons. The light is coherent and monochromatic, which is why lasers appear in nearly every interference problem.
Because interference fringes only stay sharp if the light is coherent (in phase) and monochromatic (one wavelength). A laser provides both, so the path-difference condition d sin θ = mλ produces a clean, stable pattern, like the one analyzed in the 2025 FRQ Q4.
No. Monochromatic means one wavelength; coherent means the waves keep a constant phase relationship. Laser light happens to be both, but a filtered lightbulb can be roughly monochromatic without being coherent.
No. You won't be asked about laser engineering or pumping mechanisms. You need the physics: stimulated emission between quantized energy levels produces coherent, monochromatic light, and that light creates interference patterns you can analyze.
When a laser's photons pass through two slits one at a time, each photon hits the screen at a single point, but the points build up into the bright-and-dark fringe pattern. The pattern is a probability distribution, which is the central idea of Topic 7.7.